In connection with surface treatment of metallic objects these are pickled in acidic baths, whereby in particular hydrochloric acid is generally used, in order to provide the metal object with a clean surface to which the surface treatment adheres. Hereby metal and metal oxides are dissolved from the metallic objects, which metal will be found in the pickling solutions in the form of ions, which can be recovered in the form of salts of different kinds. Most of all pickling solutions containing iron, aluminium, and zinc are present, which pickling solutions are a great problem as they can not be deposited in any arbitrary way as the metals present can create an environmental problem. Furthermore, the metals present are a resource if they should be able to be recovered.
EP-A1-0 141 313 discloses a process for recovering iron and zinc from spent pickling solutions, whereby the pickling solution is first treated with an absorption agent to eliminate any organic material present, whereupon the iron present in three valence form is first reduced using metallic iron and zinc, to two valence form, the zinc present is extracted using a complex forming solvent, whereupon zinc and iron are recovered in a traditional way. The addition of zinc is carried out in a step following the addition of metallic iron in order to precipitate heavy metals present by means of a cementation. In connection with the cementation zinc will dissolve and can then be recovered as zinc chloride. When the zinc chloride has been eliminated by means of extraction iron can be recovered as a solution of iron(III) chloride after oxidation of the two valence iron. The process is hereby based on a cementation of the heavy metals present and a solvent extraction of the zinc present by means of trioctyl amine in a mixture with kerosine/isodecanol. The use of the organic solvent is a complication as these have to be recovered, partly for cost reasons, partly for environmental reasons.
There is thus a desire for a more simplified and cost efficient process for recovering the metals present, and in particular to eliminate the need for organic solvents.